Carbonate matrix acidizing extends well effective drainage by dissolving rock and forming conductive channels (wormholes) from the wellbore. Wormholing is a dynamic process that involves a balance between the acid injection rate and reaction rate. A rotating disk is increasingly being applied in the laboratory for studying reactions between fluids and solid surfaces. The acid reaction kinetics of an acid reaction with a carbonate involves three phases: (1) the transport of H+ ions from the bulk solution to the surface of a carbonate; (2) the reaction of H+/carbonates takes place on the surface of the carbonate; and (3) the transport of the reaction products from the carbonate surface to the bulk solution. The slowest phase controls the global reaction rate.
Mass transfer limits the reaction of hydrochloric acid with carbonate rock under reservoir conditions. The mass transfer coefficient determines how fast the rock is dissolved by the acid and therefore the wormhole profile and penetration during matrix acid stimulation. The mass transfer coefficient is controlled by: (1) the fluid injection rate; and (2) the diffusion coefficient of the acid. While injection rate is easily known from the job execution, the diffusion coefficient is intrinsically a hidden parameter of the fluid and reaction conditions and is measured experimentally. Currently, the diffusion coefficient of fresh acid is used to calculate the process of wormholing. The use of a fresh acid diffusion coefficient is adequate for wormholing phenomena near the wellbore, but as the wellbore penetrates deep into the formation, the tip of the wormhole contains predominantly spent acid. Therefore, the use of a diffusion coefficient based on fresh acid may overestimate the dissolution rate.